Optical fiber preform manufacture by MCVD is known, and does not require detailed exposition. See, for instance, U.S. Pat. No. 4,217,027, and the article on Fiber Optics in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 10, pp. 514-537 (1994), both incorporated herein by reference.
Briefly, preform manufacture by MCVD involves formation of a cladding material layer on the inner surface of a substrate tube, followed by deposition of particulate core material on the cladding material. The core material is doped to have higher refractive index than the cladding material. Typically, the substrate tube is silica, the cladding material is silica, or fluorine-doped silica, and the core material is germanium-doped silica. MCVD involves flowing gaseous precursor chemicals (e.g., SiCl.sub.4, or SiCl.sub.4 and GeCl.sub.4), together with oxygen and an optional carrier gas such as He into the bore of the substrate tube. An external torch heats the tube to form a moving hot zone that causes in one pass deposition of glass particles on the tube wall and sintering of the deposited particles into a glass layer. The MCVD-formed glassy particles are frequently referred to as "soot", and this terminology will also be used herein.
MCVD is widely used in commercial practice, accounting for a significant fraction of optical transmission fiber produced worldwide. In view of this significant commercial use, it is evident that any process change that can result in lower production cost is potentially of great importance.
In many, if not all, commercial embodiments of the MCVD process, the rate-limiting step in preform production is core deposition. Thus, any process changes that shorten the time required for core deposition (or increase the amount of core glass deposited per unit time) would translate into increased throughput and therefore into lower production cost.
However, as those skilled in the art know, merely increasing the deposition rate of core material beyond currently used values (e.g., more than about 0.25 gm SiO.sub.2 /minute) typically does not result in acceptable product. Specifically, such increase typically results in bubbles in the core material, with at least some of the bubbles remaining in the core during sintering, preform collapse and fiber drawing. Of course, fiber with bubbles in the core is generally unsuitable for optical fiber communication purposes.
Thus, it would be highly desirable to be able to form bubble-free core material at a rate in excess of currently used rates. This application not only discloses the mechanism of bubble formation but also discloses process changes that make possible the deposition of bubble-free core material at rates in excess of current rates.
Undoped SiO.sub.2 or silica glass doped, in addition to Ge, with a substantial amount of one or more elements other than Ge (e.g., F or P) can be deposited by MCVD at relatively high rates (e.g., 0.6 gm SiO.sub.2 /minute) substantially without bubble formation. However, silica doped with Ge and a substantial amount (e.g., more than 1 mole %) of dopant or dopants other than Ge is generally not found in the core of conventional optical fibers, e.g., Lucent Technologies' 5D.RTM. or True Wave.RTM. fiber. The instant invention is thus primarily directed towards increasing the rate of MCVD deposition of essentially bubble-free GeO.sub.2 -containing silica-based core glass comprising at most 1 mole % of constituents other than SiO.sub.2 and GeO.sub.2, and only secondarily towards further increasing the MCVD deposition rate of essentially bubble-free GeO.sub.2 -containing silica-based glass comprising more than 1 mole % constituents other than SiO.sub.2 and GeO.sub.2.
Glossary and Definitions
By "core material" or "core glass" we mean herein material or glass that is up-doped (i.e., .DELTA.&gt;0) with Ge, typically containing at least 1 mole % of GeO.sub.2, and that contains at most 1 mole % of constituents other than SiO.sub.2 and GeO.sub.2.
By a "conventional" torch we mean herein a torch (typically oxyhydrogen torch) having a multiplicity of interleaved orifices for oxygen and hydrogen, arranged in a substantially linear array such that a relatively narrow region of a rotating substrate tube is heated. The heated region is referred to as the hot zone.
By "performance-affecting" bubbles we mean bubbles in optical fiber that measurably affect an optical fiber property, e.g., fiber strength or loss.
Closed voids in the as-deposited material (soot layer) herein are typically referred to as "pores", and uncollapsed voids in the sintered material are typically referred to as "bubbles".
".DELTA." herein has its conventional meaning, namely, (n.sub.core -n.sub.clad)/n.sub.clad, where n.sub.core and n.sub.clad are the refractive index of the fiber core and of the cladding surrounding the core, respectively.